1. Field of the Invention
The invention relates to a capacitive bridge-type probe for measuring clearance between a fixed surface and a member movable relative to said fixed surface. More specifically, the invention relates to a probe for measuring the clearance between the tip of a movable turbine blade and the inner surface of the turbine shroud.
The invention also relates to a capacitance to voltage conversion circuit for converting changes in capacitance of the capacitive bridge-type probe to a voltage. More specifically, the invention relates to such a conversion circuit which uses offset means for providing a predetermined imbalance to the bridge whereby to provide automatic calibration of the circuit.
2. Description of Prior Art
It is known in the art to use full bridge techniques for measurement of extremely small changes in relatively large quantities. A prime example of such a bridge is the strain gauge bridge. An advantage of the full bridge techniques, when properly constructed, is their inherent stability over a wide range of environmental temperature ranges. If all elements of the bridge are identical, the bridge will remain in a balanced condition over a wide temperature range which does not destroy the base properties of the materials making up the arms of the bridge. Obviously, variations will occur in each of the individual arms of the bridge as a function of temperature. However, if the arms possess identical properties, the electrical balance of the bridge will be maintained as each arm will vary by the same amount.
Thus, any electrical imbalance occurring will be purely as a result of differential changes of the arms deliberately induced by effects of the quantity to be measured. As such, bridge imbalance is a measure of a specific quantity, for example, blade tip clearance.
Full capacitive bridges are also known in the art. The present application does not relate to the idea of capacitive bridges per se, but rather, to a particular structure for a probe comprising a capacitive bridge type probe.
Electronic circuitry, for converting the change in a bridge parameter (e.g. change in a resistance-type bridge or change in capacitance in a capacitance-type bridge) are also known in the art. However, we are not aware of any such circuitry which is both sensitive only to changes in the parameter and also includes automatic calibration.